Formation of small pore SAPO-44 type molecular sieve

The hydrothermal transformation of silico-aluminophosphate gel with cyclohe xylamine to SAPO-34 has been examined. The hydrothermal crystallisation pro ducts of the SAPO have been investigated by X-ray diffraction, FTIR, nitrog en and water adsorption, thermogravimetric analysis, surface analysis and A l-27, P-31 and Si-29 MAS NMR. Structural changes were observed in the silic o-aluminophosphate gel with and without organic template and during the hyd rothermal crystallisation. The silico-aluminophosphate gel converted to pur e SAPO-CHA phase in 168 h at 473 K. The surface of SAPO-44 was silicon rich as compared with that of SAPO-34 and SAPO-18. The Al-27 MAS NMR signal of tetrahedrally coordinated Al observed in the silico-aluminophosphate gel wi thout the organic template was changed to octa-, penta- and tetrahedrally c oordinated aluminium upon the addition of the cyclohexylamine template to t he SAPO gel. After 3 h of hydrothermal treatment at 473 K however, the Al-2 7 MAS NMR Signals of the octahedral and pentacoordinated aluminium were rem oved. This was also confirmed by P-31 and Si-29 MAS NMR. The tetrahedrally coordinated P and Si were detected within 3 h at 473 K. The sorption capaci ty and adsorption-desorption trends of the SAPO gels and the crystallisatio n products were found to be different. Si-29 MAS NMR results indicated that the percentage of Si (4Al) and its distribution were significantly affecte d by the crystallization period. SAPO-44 was thermally stable up to 973 K w ith phase change observed over the calcination temperature of 1193 K. The S APO gels and the crystallisation products have also been investigated for t heir catalytic behaviour in n-hexane and ethanol conversion reactions. (C) 1999 Elsevier Science B.V. All rights reserved.